Recent Results in Seismology

Once again it is time to acknowledge that I will never read all of the papers I’ve flagged in my RSS reader… but I can at least go through the abstracts. While my summaries here may be slightly in error due to the fact that I haven’t actually read the papers in question, here is what I’m skimming:

Clay might trigger earthquakes – Many clay minerals break down, when heated, to produce H2O + different clay minerals. If this water is produced inside a clay-lined fault zone, and not allowed to leave, it might weaken the fault enough to produce an earthquake (cf. the beer can experiment). Takahashi et al. ran a set of lab experiments on gouge material from a real-life fault, and found that this effect really is large enough to be potentially significant.

Clayey soils are not safe from liquefaction – It’s pretty standard, in geotechnical engineering, to assume that soils with lots of fine clay particles will not liquefy during an earthquake. However, it’s been very difficult to test this in the field – earthquakes don’t come with enough warning to let you wire up the necessary instrumentation. Instead of using an earthquake, then, Hatzor et al. used a bomb – they wired up a field with a bunch of geophones and set off a 50 kg charge of TNT. Sand with “safe” amounts of clay turned to slurry.

Home seismometers – Japan’s meteorological agency has recently started to deliver earthquake alerts. (NB: This earthquake early warning system is not the same as earthquake prediction. Early warning systems cannot tell us when a fault will rupture in an earthquake. Once the earthquake has already started, though, an early warning system can use the fast-traveling but weak P-waves to alert people to the imminent arrival of the slower-moving but more damaging S and surface waves. That gives you a few seconds of warning, which is enough time to move away from windows and under a table; anyone who tells you that they can provide enough time for you to leave town is a quack.) Anyway, Horiuchi et al. would like to put seismometers inside all of the earthquake alarm units, instead of just connecting them to alarms on the centralized seismic network. It looks a bit like the laptop accelerometer seismic network, but presumably the instruments would be slightly higher quality, and slightly less subject to noise from cats walking across the keys.

Triggered nonvolcanic tremor – Volcanic tremor has been observed for yonks, and we think we kinda sorta know what causes it (magma sloshing back and forth in a constricted tube). The discovery of quiet, similarly reverberating signals that are not associated with volcanoes, and are definitely not sudden earthquakes, is still relatively new. There is no real consensus about what causes these signals, either: Is it water sloshing around? Slowly slipping rocks? Something else entirely? We don’t know. We do know that these tremor events can be set off by the arrival of seismic waves from distant earthquakes.

This might not be the most useful puzzle-piece quite yet – we know that normal earthquakes can sometimes be triggered by arriving seismic waves, but we don’t understand that either, so it’s a fat lot of good that does us in trying to figure out what nonvolcanic tremor really means. At least now we have a strong hint that these phenomena are, indeed, part of the same puzzle.

Comments

I thought the early warning systems worked on the principle that electrons travel faster than all seismic waves, so a first (large) detection can radio or phone in an alert before the seismic waves reaches much farther. So the epicenter guy is screwed, but the bullet train 150 km up fault has a chance to stop.